This invention relates to the production of uranium and more particularly to the elution of anionic ion exchange resins in uranium concentration procedures employing an ammonium carbonate eluant.
Uranium is produced from uranium-bearing ores by various leaching procedures which employ an alkaline or acid lixiviant to leach the uranium from its accompanying gangue material. The leaching operation may be carried out in conjunction with a surface milling operation in which the uranium ore is mined and then crushed and blended prior to leaching, heap leaching of ore piles at the surface of the earth, or in-situ leaching in which the lixiviant is introduced into a subterranean ore deposit and then withdrawn to the surface of the earth through suitable injection and production systems. Typically, the alkaline lixiviants are aqueous solutions of a suitable oxidizing agent such as oxygen or hydrogen peroxide and a leaching agent comprised of alkali metal or ammonium carbonates or bicarbonates or mixtures thereof. The pH of the lixiviant normally is within the range of about 7 to 10 at which the leaching agent functions to solubilize hexavalent uranium in the ore in the form of anionic uranyl complexes such as uranyl tricarbonate ions.
The pregnant lixiviant produced during the leaching operation is then treated in order to recover the uranium therefrom. In one conventional recovery procedure, a concentrated uranium solution is produced by passing the pregnant lixiviant over an anionic ion exchange resin and then eluting the resin with an eluant to desorb the uranium from the resin. The eluting procedure produces a relatively concentrated uranium solution in the effluent from the ion exchange column. This rich uranium solution, termed the "eluate", is then treated to precipitate uranium to produce the familiar "yellowcake". Among various eluants employed in uranium concentration are aqueous solutions of alkali metal or ammonium carbonates or bicarbonates. The eluant may also contain a suitable salt such as an alkali metal or ammonium chloride which, together with the carbonate or bicarbonate ions, functions to exchange the uranyl complex anion from the cationic adsorption sites on the resin.
The use of ammonium carbonate or bicarbonate is advantageous under certain circumstances since carbon dioxide and ammonia can be recovered from the rich eluate during the uranium precipitation process and then employed in forming fresh eluant. Thus, U.S. Pat. No. 2,811,412 to Poirier discloses the use of an aqueous solution of ammonium carbonate as an eluant. The ammonium carbonate is present in a relatively high concentration within the range of 10 to 25 percent. The uranium is recovered from the rich eluate by heating the solution, preferably to the boiling point whereby ammonia and carbon dioxide are driven off and uranium is precipitated. Poirier discloses that the ammonia and carbon dioxide released during heating of the eluate may be collected and reconverted to ammonium carbonate which is then recycled and used anew. Another uranium concentration process is disclosed in U.S. Pat. No. 4,092,399 to Narayan et al. In this procedure, the eluate is an aqueous solution of ammonium carbonate, bicarbonate, and chloride. The carbonate eluting agent is employed in relatively low concentrations of about 1 to 3 percent. During the processing step in which molybdenum is removed from the rich eluate prior to precipitation of uranium, the eluate is treated with hydrochloric acid. The carbon dioxide evolved during the acid treatment is used for elution solution makeup.